This invention relates to lasers, and in particular to a method and apparatus for providing active compensation for a semiconductor laser array. Active compensation can be used for control of temperature, wavelength, and other characteristics of each individual laser within the laser array.
Optical communications systems are used to provide high-speed communication, including voice and data services. Conventionally, only one optical signal at one wavelength (or one channel) is transmitted per optical fiber. While demand for bandwidth soars, this one optical wavelength per optical fiber design has limited the capacity of optical communications systems.
Wavelength division multiplexing (WDM) is a technology that greatly increases the information transmission capacity of optical fibers in communications systems. In a WDM system, a multiplicity of independently modulated lasers, each with a unique but precisely controlled wavelength, generate the combined optical signal that is transmitted on the optical fiber. Since multiple lasers are used to drive each fiber, it is desirable for both economic and packaging reasons to use an array of WDM lasers (i.e., one that is fabricated on a single substrate) instead of individually fabricated and packaged devices. Unfortunately, several problems can render the use of simultaneously driven WDM laser arrays impractical.
One problem with conventional WDM laser arrays is the shift in laser wavelengths caused by changes in temperature at the laser sites when one or more particular lasers are turned xe2x80x9conxe2x80x9d (i.e., selected for use) or xe2x80x9coffxe2x80x9d (i.e., deselected). This change in temperature is generally not caused by the bit pattern of the data used to modulate the laser. Typically, the data bit rate is high and results in an average duty cycle of approximately fifty percent and a steady-state power dissipation of approximately one half peak power. The modulating data have negligible effects on the laser temperature.
The problematic temperature change generally occurs when one or more lasers is xe2x80x9cdeselectedxe2x80x9d (i.e., the data stream input to the laser is removed). This deselection changes the steady-state power of the deselected laser to a small, or approximately zero, value. Specifically, the problem arises because the operating wavelength of semiconductor lasers is dependent on temperature. For WDM lasers, the operating wavelengths are usually specified to within a narrow range and it is typically necessary to control their operating junction temperatures to within a small temperature range (e.g., xcex94Txe2x89xa60.1xc2x0 C.) in order to insure operation within the allowable wavelength range. Unfortunately, because of the substantial power levels at which WDM lasers typically operate (e.g., PL=0.2 to 0.5 watts per laser), when one or more lasers in the array are deselected, the thermal crosstalk between these devices in practical array packaging configurations leads to temperature changes much greater than the allowable temperature range. This problem makes it extremely difficult to use laser arrays in applications (e.g., WDM) involving simultaneous and selectable operation of lasers in the array. In fact, for many applications, this problem can preclude the use of laser arrays altogether.
Another problem with conventional WDM laser arrays relates to manufacturing yields. As described above, each laser in the WDM laser array is operated within a narrow wavelength range. For individually packaged lasers, the tight wavelength specifications can be addressed by incorporating a thermoelectric (TE) cooler and temperature controller (or TE cooler/controller) that can be used to adjust the temperature of the laser to obtain the specified operating wavelength. However, with an array of lasers sharing only one TE cooler/controller, the temperature of each of the lasers cannot be independently adjusted to obtain the specified operating wavelengths for all lasers. The probability that the wavelengths of all lasers in the array can be adjusted to be within their wavelength specifications at a particular common temperature controller setting is drastically reduced in comparison to the independently temperature compensated laser. This translates to poor manufacturing yields and increased costs for WDM laser arrays, as compared to individually packaged devices. The poor manufacturing yields make the use of laser arrays prohibitively expensive.
For the foregoing reasons, techniques that provide temperature or wavelength compensation for a semiconductor laser array are highly desirable.
The invention provides techniques for active compensation for a semiconductor laser array. Active compensation can be used for control of temperature, wavelength, and other characteristics of the lasers within the laser array. Through active compensation, operating performance and manufacturer yields can be improved.
A laser array according to the invention includes a plurality of lasers and a plurality of dissipation elements. In one embodiment, the dissipation elements are interstitial to the lasers (i.e., located between the lasers and at both ends of the laser array). In one embodiment, the dissipation elements are implemented as non-lasing diodes. The dissipation elements are selectively activated (i.e., turned xe2x80x9conxe2x80x9d to dissipate power) to maintain the required temperature at the laser junctions. This allows the lasers to operate at their specified wavelengths. The dissipation elements can be individually controlled and two or more bits of resolution for the control can be provided.
In one application, active compensation is used to adjust (i.e., to compensate) the temperature of selected lasers (i.e., lasers that are turned xe2x80x9conxe2x80x9d) when one or more lasers are deselected (i.e., turned xe2x80x9coffxe2x80x9d). In another application, active compensation is used to adjust (i.e., xe2x80x9ctweakxe2x80x9d) the wavelengths of the lasers within the laser array to be within their wavelength specifications. Active compensation can be performed dynamically during operation of the laser array.
One specific embodiment provides a semiconductor laser array having active compensation. The laser array includes a plurality of lasers fabricated on a semiconductor substrate and a plurality of dissipation elements located within the laser array. At least one of the dissipation elements has independent control (i.e., independent from a package cooler).
Another specific embodiment provides a compensation circuit for maintaining an operating characteristic of lasers within a laser array. The laser array includes a plurality of lasers and a plurality of dissipation elements. The compensation circuit includes a table to store compensation values and drive circuits coupled to the table. The drive circuits receive the compensation values and generate drive signals for the dissipation elements.
Another specific embodiment provides a laser transmitter having active compensation. The laser transmitter includes a laser array and a compensation circuit. The laser array includes a plurality of lasers fabricated on a semiconductor substrate and a plurality of dissipation elements located within the laser array. At least one of the dissipation elements has independent control. The compensation circuit couples to the laser array and includes a table to store compensation values and drive circuits. The drive circuits couple to the table to receive the compensation values and generate drive signals for the dissipation elements.
Another specific embodiment provides a method for maintaining an operating characteristic of lasers within a laser array. The method includes determining an operating state of each laser in the laser array, selecting compensation values from a table corresponding to the determined operating state of the laser array, generating drive signals in accordance with the compensation values, and providing the drive signals to selective ones of the dissipation elements within the laser array.
Yet another specific embodiment provides a method for adjusting operating wavelengths of lasers within a laser array. The method includes determining the operating wavelengths of the lasers under an initial operating conditions, computing wavelength errors based on the determined operating wavelengths and the specified wavelengths, determining compensation values that reduce the wavelength errors, generating drive signals in accordance with the compensation values, and providing the drive signals to selective ones of the dissipation elements within the laser array.
The foregoing, together with other aspects of this invention, will become more apparent when referring to the following specification, claims, and accompanying drawings.